A 287.74-mcd-thick sediment sequence was recovered at Site 1242, spanning the interval from the middle Miocene to the Holocene with a prominent hiatus from ~2.5 to 12.0 Ma that divides the sequence into two lithologic units (Table T8; Fig. F13). Unit I spans the upper Pliocene to Holocene and contains hemipelagic sediment that consists of nannofossil clays and clayey nannofossil oozes. Siliciclastic components are dominated by clay minerals with minor silt-sized minerals (Fig. F14), primarily feldspars and quartz, throughout the unit. Nannofossils are the dominant biogenic component (Fig. F15). Foraminifers are present in variable minor amounts, and the contribution of siliceous microfossils is generally low except for a distinct interval of diatom-bearing sediment between ~100 and 140 mcd. Authigenic components include micrite, dolomite, and pyrite. Volcanic glass is frequently present as a minor component of the dominant lithologies in Unit I. Forty-five ash layers are observed in Unit I, eighteen of which are correlative between holes.
Physical properties measurements within Unit I are most strongly influenced by changes in calcium carbonate and siliciclastic content. GRA bulk density and lightness (L*) gradually increase downcore to ~200 mcd, consistent with gradually increasing calcium carbonate. Magnetic susceptibility (MS) shows slightly higher values in intervals containing higher siliciclastic content, and the broad pattern of NGR is similar to that of the siliciclastic content (Fig. F13).
A hiatus representing ~9.5 m.y. occurs at the base of Unit I, and an abrupt change in lithology accompanies the transition to Unit II. Lithologic Unit II contains a short sequence of middle Miocene pelagic sediment dominated by diatom-bearing nannofossil oozes. Except for minor abundances of clay minerals, Unit II lacks siliciclastic components. This interval is characterized by lower MS and higher GRA bulk density.
Site 1242 represents a homogeneous hemipelagic sedimentary record within an active continental margin setting spanning the upper Pliocene and the complete Quaternary (Unit I). Long-term decreasing biogenic contents throughout this interval may indicate increasing terrigenous supply, possibly related to the uplift of the Central American cordillera and/or long-term climate change. Rhythmic changes in MS are likely associated with relative variations in biogenic and terrigenous input, reflecting millennial- and orbital-scale changes in productivity, ocean circulation, and climate.
The Miocene sediments of Unit II represent pelagic conditions prior to the closure of the Isthmus of Panama. The ~9.5-m.y. hiatus between Units I and II, which spans the middle Miocene to upper Pliocene, is coeval with the closure of the Isthmus of Panama. Relatively high contents of siliceous microfossils may reflect higher Miocene productivity associated with the tectonic backtrack of Site 1242 on the Cocos plate into the active equatorial upwelling zone.
Two lithologic units were defined at Site 1242 on the basis of visual core description, smear slide analysis, MS, color reflectance, NGR, moisture and density (MAD), and GRA bulk density measurements (Table T8; Fig. F13).
The major lithologies of Unit I are nannofossil clays and clayey nannofossil oozes. Siliciclastic components are dominated by clay minerals with minor silt-sized and rare sand-sized minerals throughout the unit (Fig. F14). Sediment color is homogeneous with only subtle gradational changes between different hues of grayish olive green. Moderate bioturbation is evidenced by the presence of mottles, burrow fills, and Zoophycos traces. Foraminifer fragments and spicule aggregates were commonly observed on the split core surface and sometimes as burrow infill (Fig. F16). Distinct dark layers of the pennate diatom Ethmodiscus are present from ~205 to 215 mcd (Fig. F17). Minor features in Unit I include wood fragments, microfaults, and a fold at ~142-143 mcd that may indicate a slump. Black coarse grains composed of pyrite-rich clay aggregates and glauconite are present toward the base of Unit I (~250 mcd).
Siliciclastic content ranges from ~20% to 60% throughout Unit I (Fig. F14). Abundance increases significantly from ~10 to 25 mcd, generally decreases downhole to ~180 mcd, and then remains comparatively low to the bottom of the unit. Coarser-grained siliciclastics, primarily feldspars and quartz, vary downcore with abundances between ~2% and 6% (Fig. F14).
Nannofossils are the dominant biogenic component, ranging from ~30% to 70% (Fig. F15). A strong downhole decrease in the uppermost ~25 mcd corresponds to increasing siliciclastic content. Farther downhole, nannofossil abundance generally increases, with broad minima paralleling the increases in micrite. Foraminifers are present in variable minor amounts, primarily in the range of ~4%-12%, except for maximum values of up to >20% near ~155 mcd. Diatom contents are generally only a few percent but reach ~10% between ~100 and 140 mcd. In this interval, radiolarians, silicoflagellates, and spicules also approach a maximum of ~2% but are present otherwise in very small amounts only (Fig. F15).
Authigenic components include micrite, dolomite, and pyrite. Micrite abundance generally increases downhole with two broad maxima, one between ~100 and 150 mcd and a second between ~210 and 250 mcd (Fig. F15). Dolomite is present as a minor component (~4%-9%) in a short interval from ~264 to 275 mcd. Pyrite abundance varies throughout the unit, with maximum values between ~200 and 250 mcd.
The only notable minor lithologies within Unit I are volcanic ash layers. Forty-five ash layers are observed in Unit I, eighteen of which are correlative between holes (Table T9). The ashes range from light gray to brownish gray and black and are usually characterized by sharp basal contacts and diffuse upper contacts (Fig. F18). Ashes at Site 1242 contain mostly silt- and sand-sized clear platy and vesicular glass, and accessory minerals include plagioclase feldspar, orthopyroxene, pyrite, quartz, hornblende, and biotite. Ash layers are present throughout the uppermost ~278 mcd, with high-frequency intervals between ~30 and 120 mcd and ~160 and 210 mcd. In addition to these discrete layers, volcanic glass often is present as a minor component (~2%-10%) of the dominant lithology throughout Unit I (Fig. F14).
Color reflectance measurements from Unit I plot in the "green" (a* < 0) domain of the a*-b* color plane (Fig. F19). Lightness increases downhole throughout the unit (Fig. F13), generally paralleling the decreasing siliciclastic and increasing calcium carbonate concentrations. Magnetic susceptibility is high at the top of Unit I (>40 instrument units). Below a sharp drop at ~1.5 mcd, MS averages ~10 to 15 instrument units, with pronounced variability on meter to decimeter scales (Fig. F13). Average values decrease below ~150 mcd, coinciding with increasing calcium carbonate concentrations (Fig. F15). Distinct maxima of >25 instrument units mostly reflect volcanic ash layers. Changes in NGR seem to parallel changes in siliciclastic content, particularly in the uppermost ~25 mcd (Fig. F13). GRA bulk density generally increases downhole (Fig. F13), reflecting the gradual increase in calcium carbonate and the simultaneous decrease in siliciclastic content. In addition, this increase may be related to compaction and lithification in the lower portion of the unit. A pronounced step change in GRA bulk density at ~200 mcd coincides with the change from APC to XCB coring. Throughout Unit I, GRA bulk density and MAD discrete bulk density are fairly well correlated to each other (r2 = 0.70). APC cores alone would probably yield a better correlation, but the XCB cores, which lie on a similar slope, introduce a constant offset, thus reducing the correlation. Porosity mirrors changes in bulk density (r2 = 0.94) (Fig. F20).
A prominent hiatus at ~280 mcd represents the transition to lithologic Unit II. The hiatus is characterized by a very sharp color change from grayish olive green to very pale brown (Fig. F21). Lithologic Unit II contains pelagic sediment composed of diatom-bearing nannofossil ooze with varying abundance of foraminifers, spicules, radiolarians, and silicoflagellates. Sediment color in Unit II varies from very pale brown to brown. Zoophycos traces and burrow structures are present throughout Unit II, indicating moderate bioturbation.
The primary components of Unit II are nannofossils (~60%-70%) and diatoms (~10%-20%) (Fig. F15). Foraminifers and other siliceous microfossils (spicules, radiolarians, and silicoflagellates) represent minor biogenic components with abundances ranging from ~1% to 10%. Micrite abundance ranges from 0% to 10% in Unit II (Fig. F15). Siliciclastic content is much lower (<10%) than in Unit I (Fig. F14).
All physical properties measurements show an abrupt change at the beginning of Unit II (Fig. F13). MS and NGR decrease significantly, and lightness increases. These changes coincide with a significant decrease in siliciclastic content and an increase in biogenic content. GRA bulk density also decreases, which is consistent with the significant increase in biogenic silica content observed in Unit II. Color reflectance measurements from Unit II plot in the "reddish" (a* > 0) domain of the a*-b* color plane (Fig. F19).
A 287.74-m-thick sediment sequence was recovered at Site 1242, spanning the interval from the middle Miocene to the Holocene with a prominent hiatus (~2.5-12 Ma) that divides the sequence into two lithologic units deposited within very different depositional environments. Today, this site is located within a tectonically active graben containing sediment of late Pliocene to Holocene age (lithologic Unit I). This graben overlies a sequence of Miocene and older sediment that represents lithologic Unit II.
The Miocene sediment of Unit II represents pelagic conditions before the closure of the Isthmus of Panama. The relatively high abundance of siliceous microfossils in Unit II suggests higher Miocene productivity that may be related to the southward tectonic backtrack of Site 1242 on the Cocos plate (Pisias et al., 1995) toward the equatorial upwelling zone. A comparison of the backtrack paths of Sites 1241 and 1242 shows that at ~12 Ma, Site 1242 was located in a setting similar to the Site 1241 Pliocene setting. This tectonic observation is further supported by the similar lithologies observed for these periods at the two sites (see "Lithostratigraphy" in the "Site 1241" chapter).
The ~9.5-m.y. hiatus, which spans the middle Miocene to upper Pliocene, is coeval with the closure of the Isthmus of Panama, suggesting a plausible connection to this event. Additionally, during this interval, the Cocos Ridge at Site 1242 was approaching the Central American subduction zone, causing increasing tectonic extension, which probably formed the local graben structure that contains upper Pliocene to Holocene Unit I. Hence, both local tectonics and the large-scale isthmus closure might explain the possible lack of sediment accumulation or the tectonic "removal" of the missing upper Miocene to lower Pliocene sedimentary sequence.
Unit I contains a fine-grained, homogeneous hemipelagic sedimentary record with moderate to high sedimentation rates ranging from ~6 to 20 cm/k.y. (see "Age Model and Mass Accumulation Rates") spanning the upper Pliocene to the Holocene. Aside from a possible slump observed at ~143 mcd in one hole, there is no widespread lithologic evidence for sediment redeposition at Site 1242, suggesting a complete and continuous sedimentary sequence representing the last 2.5 m.y.
The depositional environment was probably similar to the modern setting (i.e., the location of Site 1242 at a depth of ~1370 m on the northeastern Cocos Ridge), ~30 km off the Central American active continental margin under relatively low salinity and nutrient-poor surface waters. The upper Pliocene is characterized by high calcium carbonate and relatively low siliciclastic content, followed by a gradual decrease in calcium carbonate and increase in siliciclastics into the upper Pleistocene-Holocene that may be attributed to one or more of the following:
Preliminary sedimentation rate estimates (see "Age Model and Mass Accumulation Rates") suggest relatively low values of ~6 cm/k.y. for the uppermost ~20 mcd, where carbonate content is lowest and siliciclastic content is highest. This may imply that terrigenous sediment input is affected by local synsedimentary processes.
Magnetic susceptibility displays significant variability on decimeter to meter scales throughout the upper Pliocene to Holocene sequence, suggesting changes in the relative supplies of terrigenous and biogenic material to Site 1242. These changes, in part, may reflect millennial- to orbital-scale changes in productivity and/or climate.
Increased ash layer frequency between ~30 and 120 mcd and ~160 and 210 mcd indicates increased volcanic activity from ~0.4 to 1.1 Ma and from ~1.3 to 1.6 Ma, respectively, which is consistent with high ash accumulation between ~0 and 2.5 Ma in the Caribbean Sea (Cadet et al., 1982; Sigurdsson et al., 2000). Clear platy and vesicular glass and intermediate accessory mineral compositions suggest an andesitic volcanic source, most likely in Central America.